A common method for connecting structures is by mechanical fastening. Mechanical fastening includes connecting a first structure 10 to a second structure 12 directly using mechanical fasteners 14, as shown in FIG. 1, or connecting the first structure 10 to a second structure 16 indirectly by way of one or more intermediate components 18 and mechanical fasteners 14, as shown in FIG. 2. Although mechanical fastening is often acceptable in terms of cost, weight, complexity and strength, fastening methods having still lower costs, complexity, and weight, without compromising strength, are always desired.
One substitute for mechanical fastening is adhesive bonding. FIG. 3 shows a traditional adhesive joint, generally designated by reference number 30, including a double lap joint element 32 (also referred to as a clevis or “pi” joint element) having a slot 34. The joint element 32 is connected to a first structure 36 and bonded to the second structure 38 by an adhesive 40. A plug or nugget 42 of adhesive fills a bottom 44 of the slot 34. Fastening by adhesive bonding is generally less costly than mechanical fastening because it does not require formation of holes usually required for mechanical fastening.
Although adhesively bonded joints are not necessarily lighter than mechanically fastened joints, in many cases they are. This difference often results from the difference in materials required to form the joints. Another reason for weight difference results from differences in the structures being joined. Allowable structural dimensions often depend on the type of joint used. For example, although strength requirements of a first structure having holes for mechanical fastening may mandate a certain thickness of the first structure, the same requirements may allow a smaller thickness when no holes are present.
Although adhesive bonding is generally simpler and less costly compared to conventional mechanical fastening, loss of structural integrity has been linked to the presence of nuggets 42 in the bottom 44 of the slot 34. Specifically, most adhesives 40 experience some degree of relative contraction with respect to the joint element 32 and second structure 38 during curing. Also, because adhesives 40 generally have a coefficient of thermal expansion greater than that of the joint element 32 and second structure 38, the adhesive expands and contracts with respect to the joint element and second structure during thermal cycling (exposure to relatively high and relatively low temperatures).